1. Field of the Invention
The present invention relates to wireless communication transceivers and modems and, more particularly, to the construction of a multi-frequency, multi-channel transceiver system such as might be used in a multi-user base station for terrestrial cellular or fixed-wireless applications.
2. Description of the Related Art
Directional antennas are widely used in a variety of communications systems to more efficiently transmit and receive radiated signals. Relative to an isotropic antenna, which transmits and receives signals equally in all directions, a directional antenna has an antenna gain pattern that is greater in certain directions than others, typically having a higher-gain main lobe several degrees wide (i.e., an antenna beam). Generally, a greater antenna gain reduces the amount of power required to transmit and receive signals between two communication devices. Thus, steering the main lobe of a transceiver's antenna gain pattern in the direction of another communication device facilitates communication between the two devices.
To be useful in certain applications, it may be necessary to rapidly point the antenna beam of a directional antenna in different directions. For example, base stations employed in cellular or wireless communication systems are required to communicate with several mobile communication devices at once. Often, the cell or region covered by the base station is divided into angular sectors (e.g., three 120° sectors), with certain antennas being responsible for communications with any mobile communication devices in a given sector. To permit virtually instantaneous redirecting of the antenna beam within the sector, an antenna formed of a phased array of independently controllable antenna elements may be used. The antenna beam is formed by applying appropriate phase and gain to the individual elements in the array.
More specifically, beamforming is a type of spatial filtering in which an array of sensor elements is controlled with appropriate signal processing to implement a phased array antenna for the purpose of shaping the antenna response over time in a space-varying manner (i.e., steering gain in some directions, while producing attenuation or nulls in other directions). In a radio communications system, a signal arriving at each element of an antenna array will arrive at slightly different times due to the direction of arrival with respect to the antenna array plane (unless the signal has normal incidence to the plane, in which case the signal will arrive at all elements simultaneously). A phased-array receive antenna achieves gain in a particular direction by phase shifting, or time shifting, the signal from each element, and then summing the phase-shifted element signals in a signal combiner. By choosing the relative phasing of each element appropriately, coherence can be achieved for a particular direction of arrival (DOA), across a particular signal bandwidth.
Digital beamforming is analogous to analog beamforming, except that the received signal on each antenna element is independently digitized, and the phasing/combining operation is performed mathematically on the digital samples. The present inventors describe digital beamforming techniques in U.S. patent application Ser. No. 09/778,854 entitled “Integrated Beam Forming/Rake/MUD CDMA Receiver Architecture”, filed Feb. 8, 2001, the disclosure of which is incorporated herein by reference in its entirety. Conventionally, digital beamforming is done on a wideband signal, prior to despreading a CDMA waveform. This forces the computationally intense beamforming to take place at a much higher sampling rate, resulting in more mathematical operations per second, and corresponding increased hardware cost. To address this shortcoming, beamforming can be performed at baseband, as disclosed by Hanson et al. in U.S. Pat. No. 6,052,085, the disclosure of which is incorporated herein by reference in its entirety.
Furthermore, digital beamforming is conventionally performed as a separate process, independent of symbol modulation/demodulation, perhaps even as a separate product from the modem. In addition to the resulting inability to support advanced demodulation techniques with this architecture, the cost of the beamforming function is greater as a stand-alone function, compared to the incremental cost of adding the capability to a modem. The largest cost-component of beamforming is the complex multiplication of each sample for each element with the beamforming weights. Thus, whether stand-alone beamformers merely point in the direction of the signal of interest, or respond more adaptively to dynamic interference conditions by null-steering, such beamformers still lack the ability to be tightly coupled with potential advanced demodulation techniques.
When combined with the modem, there is potential to absorb the complex multiply required for beamforming into computation already taking place for extremely low incremental cost. In U.S. Pat. No. 5,764,187, the disclosure of which is incorporated herein by reference, Rudish et al. disclose an implementation of beamforming using digital direct synthesis (DDS) functions. However, Rudish does not suggest or recognize potential hardware and processing savings in both signal transmission and reception. Specifically, Rudish does not suggest combining demodulation with beamforming or using hardware in a time-shared manner for both transmit and receive. Rudish's architecture is highly parallel and does lend itself to time multiplexing techniques which could potentially reduce hardware requirements.
Accordingly, there remains a need for an efficient, integrated way of incorporating beamforming technology, for both transmit and receive, into base stations or transceiver terminals that process large number of users simultaneously using time division multiple access (TDMA) and/or frequency division multiple access (FDMA) technology. This problem can be extremely computationally burdensome, and architectures for cost-effectively performing this processing are not addressed sufficiently in the prior art.